The present invention relates to the field of photoplethysmography and, more specifically, to an improved system and method for determining sensor attributes. The invention is particularly apt for use in pulse oximetry applications to identify compatible sensors and/or otherwise to provide for the transfer of calibration and of other information between sensors and other system components.
In the field of photoplethysmography light signals corresponding with two or more different centered wavelengths may be employed to non-invasively determine various blood analyte concentrations. For example, blood oxygen saturation (SpO2) levels of a patient""s arterial blood may be monitored in pulse oximeters systems by measuring the absorption of red and infrared light signals. The measured absorption data allows for the determination of the relative concentration of reduced hemoglobin and oxyhemoglobin and, therefore, SpO2 levels, since reduced hemoglobin absorbs more light than oxyhemoglobin in the red band and oxyhemoglobin absorbs more light than reduced hemoglobin in the infrared band, and since the absorption relationships of the two analytes in the red and infrared bands are known. See e.g., U.S. Pat. Nos. 5,934,277 and 5,842,979.
Pulse oximeters systems typically comprise a disposable or reusable sensor that is releasably attached to a given patient""s appendage (e.g., finger, ear lobe or the nasal septum) for a given patient monitoring procedure and include at least one red light source and one infrared light source. The light sources are focused though a patient""s tissue and the unabsorbed light that passes through is measured to determine blood analyte concentrations.
As may be appreciated, in order to accurately compute blood analyte concentrations utilizing a given sensor, it is important that information regarding the sensor be known; for example, the center wavelengths of the light sources employed. A number of approaches have been developed for identifying sensor attributes to pulse oximeter monitors. By way of primary example, many sensors contain an electrical component having a characteristic(s) that may be measured by a pulse oximeter monitor when the sensor is interconnected thereto. Once the characteristic(s) is known, the monitor may determine what center wavelengths correspond with the sensor light sources, for example, by using a stored look-up table or correlation function. In turn, an appropriate calibration value can be utilized in determining blood analyte concentrations. Generally, the information in a stored look-up table or utilized to formulate a correlation function is based on data that corresponds with sensors originating from a known source. Such sources tend to approved by the monitor manufacturers and provide sensors and corresponding data that has been determined and verified through actual clinical use such that the sensors may be used with a high level of confidence. Increasingly, however, sensors are being offered for use with pulse oximeter monitors from additional sources which may, for example, utilize the same or similar identifying means as sensors from known sources while not necessarily utilizing light sources that have center wavelengths as the sensors from the known sources, thereby presenting potential difficulties in assuring accurate performance of the monitor/sensor combinations.
In light of the foregoing, a primary objective of the present invention is to provide a further improved approach for obtaining photoplethysmographic sensor information.
A related objective of the present invention is to provide for increased photoplethysmographic sensor information in a manner that does not increase sensor complexity.
Yet a further objective of the present invention is to provide for the communication of photoplethysmographic sensor information in a manner that facilitates enhanced reliability.
One or more of the above objectives and additional advantages are indeed realized by the present invention, wherein the disclosed photoplethysmographic system and method provides for the obtainment of at least one data value from a photoplethysmographic sensor in a two-mode process. In one aspect a photoplethysmographic system is provided that comprises: a signal generation means, a sensor identifying means and a processing means. More particularly, the signal generation means is able to provide at least one interrogation signal in two distinct modes of operation to the sensor identifying means. The sensor identifying means is operable to receive at least a first interrogation signal in two distinct modes of operation, wherein the interrogation signal is initially applied with a first polarity and then applied with an opposite polarity. The sensor identifying means is further operable to produce at least one output value for each mode of operation. The outputs produced by the sensor identifying means in response to the application of the two-mode interrogation signal may then be used by the processing means to determine sensor data.
By way of example, such sensor data may serve to identify a given sensor to a pulse oximetry monitor, wherein the monitor is enabled/disabled or otherwise calibrated for operation with the interconnected sensor. As will be appreciated, in conventional applications of the invention the signal generation means and processing means may be located at a pulse oximeter monitor, while the sensor identifying means may be located at a given cable interconnected thereto.
The signal generation means may further comprise a means for establishing the first and second modes of operation, wherein the interrogation signal may be applied in two distinct modes to a terminal of the sensor identifying means. For example, in the first mode, the establishing means may provide an interrogation signal to a sensor terminal with an initial polarity, while in a second mode the same interrogation signal may be applied to the same sensor terminal with an opposite polarity. The establishing means may be configured such that it automatically applies the interrogation signal in the two modes of operation when a sensor is attached to a pulse oximeter monitor. In one embodiment, the establishing means may comprise a power supply, an electrical storage means and a switching means. More particularly, the power supply may be operable to both provide an initial polarity to the sensor terminal and to provide power to charge the electrical storage means. For example, a power supply, such as a voltage divider, may supply a steady voltage to charge an electrical storage means and provide an initial interrogation signal with a steady state voltage.
With regard to the electrical storage means, an electrical potential may be stored from the power supply that may be selectively released by the switching means to change the system, for a predetermined time, from the first mode of operation to the second mode of operation. Releasing the stored electrical potential may cause the electrical operation in the system to be altered from a steady state operation to a transient condition. As will be appreciated, if the sensor identifying means is electrically connected to the signal generation means when the electrical operation is altered, the interrogation signal as applied to the sensor identifying means may also be altered, allowing for a second output reading to be taken during this altered state. For example, by selectively grounding a stored electrical potential, where the storage means is a charged capacitor, may cause an electrical imbalance in the signal generating system while the capacitor discharges. While discharging, the capacitor may pull electrical voltage from all electrically attached components, thus reversing the current flow and the polarity of the voltage as seen in the attached components. Typically, a processor will operate the switching means to selectively discharge the electrical storage means and change the system from the first mode of operation to the second mode of operation.
With regard to the sensor identifying means, one or more electrical components may be advantageously connected between a first and a second sensor terminal to produce output values in response to interrogation signals. The electrical components may be arranged in a manner such that the application of a single interrogation signal in two modes of operation (e.g., positive polarity and negative polarity) will produce two different output values. For example, an identifying means may comprise a simple resistor and a diode connected in parallel between the two sensor terminals; by applying a known voltage across the terminals such that the diode is reverse-biased and by measuring the resulting voltage drop, the size of the resistor can be determined. By correlating the voltage drop and/or the resistor size with predetermined sensor data tables, characteristics of the currently attached sensor can be determined. By reversing the interrogation signal""s polarity such that the diode is forward biased, a second measurement can be made across the sensor terminals that will generally be different from the first output value since most of the current will pass through the diode. This second output may be correlated with additional predetermined sensor tables to provide additional sensor specific information. As will be appreciated by those skilled in the art, numerous arrangements of electrical componentry are operable to produce different outputs when the componentry is forward biased and when it is reversed-biased. Typically, pluralities of electrical components are required to produce separate output values in response to an interrogation signal applied with two polarities. Further, one of the electrical components will generally be an active component (e.g., components whose response differs in relation to the direction or magnitude of signals presented thereto), such as a diode, in order for the sensor identifying means to produce multiple outputs.
As noted, the processing means will generally be located at a monitor that will receive the outputs generated by the sensor identifying means in the two modes of operation. Additionally, the processor may be operable to measure the response of the sensor identifying means to the application of the interrogation signal in the two modes of operation. For example, in a first mode of operation (e.g., a steady state mode), the processor may take a first measurement of the sensor identifying means"" response to the interrogation signal. When the system is switched to the second mode of operation, the processor may measure the sensor identifying means"" response to the interrogation signal once or multiple times. If the second mode of operation is a transient mode of operation, the sensor identifying means"" response may vary over time such that multiple readings may be taken which define a time/response profile. This time/response profile may, for example, record the variation of the voltage across the sensor identifying means from a first point in time to a second point in time. The monitor may then compare these responses, either singly or in combination, against stored data values and/or profiles. By way of example, the monitor may use the first response/output (e.g., a voltage value) to determine if an interconnected sensor is a sensor or a class of sensors that is recognized by the system (e.g., compare the voltage value to a set of stored voltage values corresponding to a known sensor/class of sensors) and accordingly enable or disable the monitor. The monitor may then use the second response/output (e.g., compare a second voltage reading to a second stored data value) to obtain additional information regarding the sensor (e.g., the particular type of sensor from a class of sensors, calibration data etc.) that may be used to further adjust the operation of the system.
As will be appreciated, since the interrogation signal""s polarity is reversed as applied to the sensor identifying means, a single steady state electrical signal may be applied in what amounts to two interrogation signals, one with positive polarity and one with negative polarity, thus allowing for multiple sensor outputs from a steady state signal. Though discussed in reference with a single steady state interrogation signal, it will be appreciated that if more than one interrogation signal is used (e.g., 5 volts and 10 volts) multiple outputs may be obtained for each interrogation signal. Additionally, the system may be operable to generate multiple outputs in response to variable interrogation signals.
In another aspect of the present invention, a method is disclosed to read at least one data value from a photoplethysmographic sensor in a two-mode process. After releasably interconnecting a sensor to a photoplethysmographic monitor wherein the sensor includes first and second sensor terminals and an identifier means electrically coupled between the first and second sensor terminals, a first interrogation signal is applied to the first sensor terminal with an initial polarity to obtain a first output. Then the interrogation signal polarity is reversed such that it is applied to the first sensor terminal with an opposite polarity to obtain a second output. Last, the first and second outputs are employed to identify sensor characteristics to the photoplethysmographic monitor.
The step of reversing may further entail charging an energy storage means with the interrogation signal initial to produce a stored electrical potential and utilizing this stored electrical potential to selectively reverse the interrogation signal""s polarity as applied to the first sensor terminal for a predetermined time. The initial interrogation signal may comprise a steady state electrical signal, such as a constant voltage, that may produce a steady state condition across the sensor identifying means. Accordingly, this steady state condition may be measured as a first output reading. Releasing the stored electrical potential on the system may then produce another mode of operation in which the polarity of the interrogation signal is reversed as applied to the first sensor terminal. During this period, at least a second condition, such as a transient response, may be produced across the sensor identifying means; accordingly, a second or multiple measurements may be taken during this period to obtain a second output.
As may be appreciated, employing the first and second outputs may include the sub-steps of first comparing a first data value corresponding with the first output (e.g., a first measured voltage drop) with a first predetermined data range and, second, comparing a second data value corresponding with the second output value (e.g., a second measured voltage drop) with a second predetermined data range. In one arrangement, if either of such comparisons indicate a data value outside of the corresponding predetermined range, the method may further provide for an output to a user (e.g., via a display) indicating that the interconnected sensor is not intended for use with the monitor. Alternatively and/or additionally, the monitor may be automatically disabled for use with the interconnected sensor. Last, when one or more output values are within the predetermined data ranges, the values can be used alone or in combination to select calibration information for use with the sensor.
Additional aspects and corresponding advantages of the present invention will be apparent to those skilled in the art upon consideration of the further description that follows.